topic 7/17 - equilibrium

Question 1

describe when a state of equilibrium is reached in a closed system

Answer 1

when the forward and reverse reactions are occurring at the same rate and the concentrations of products and reactants remain constant

Question 2

what is a closed system?

Answer 2

a sealed container

Question 3

define homogeneous equilibrium

Answer 3

the equilibrium is for materials all in the same state/phase (ie gases)

Question 4

define Le Chatelier’s Principle

Answer 4

the position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions

Question 5

describe the effect of a temperature increase on equilibria

Answer 5

if exothermic:
- the equilibrium will shift in the endothermic direction (to the left)
- yield of products decreases

if endothermic:
- the equilibrium will shift in the endothermic direction (to the right)
- yield of products increases

Question 6

describe the effect of a pressure increase on equilibria

Answer 6

the equilibrium will shift to the side with the least amount of moles to oppose the change in pressure.

Question 7

describe the effect of an concentration increase on equilibria

Answer 7

increase concentration of reactants - the equilibrium will shift towards the products (to the right) to oppose the change

increase concentration of products - the equilibrium will shift towards the reactants (to the left) to oppose the change

Question 8

give the equation for the synthesis of ammonia

Answer 8

N2 (g) + 3H2 (g) <-> 2NH3 (g)

Question 9

what is the compromise that needs to be achieved in the ammonia production industry?

Answer 9

conditions are often a compromise between best equilibrium yield and fastest rate to give a cheap product

Question 10

describe the effect of a catalyst on equilibria

Answer 10

a catalyst increases the rate of reaction but has no effect on the equilibrium position as it increases the rate of the forward and reverse reactions equally. However, equilibrium is reached more quickly.

Question 11

define the law of chemical equilibrium

Answer 11

at a given temperature the ratio of the concentration of products (raised to the power of their molar coefficients) to the concentration of reactants (raised to the power of their molar coefficients) is a constant.

Question 12

give the equation for Kc:

mA + nB <-> pC + qD

Answer 12

Kc = [C]^p[D]^q/[A]^m[B]^n

Question 13

significance of a small value of Kc (smaller than 1)

Answer 13

• higher concentration of reactant than product present at equilibrium
• equilibrium lies to the left hand side

Question 14

significance of a large value of Kc (greater than 1)

Answer 14

• higher concentration of product than reactant present at equilibrium
• equilibrium lies to the right hand side

Question 15

define heterogeneous equilibria

Answer 15

the substances involved are not all in the same state

Question 16

why are any solid chemicals not included in the Kc expression?

Answer 16

Kc refers to concentrations in mol dm3 and the concentration of a solid does not change

Question 17

how do you find the units of Kc?

Answer 17

you derive them

Question 18

effect on Kc of:
- reversing the reaction
- halving the coefficients
- doubling the coefficients
- summing equations

Answer 18

• 1/Kc
• square root of Kc
• square of Kc
• Kc1 x kc2

Question 19

effect of increasing temperature on the value of Kc for an exothermic reaction

Answer 19

Increasing the temperature drives the reaction in the reverse direction, increasing the reactant concentration and reducing the product concentration.
The vaue of Kc decreases correspondingly.

Question 20

effect of decreasing temperature on the value of Kc for an exothermic reaction

Answer 20

Decreasing the temperature drives the reaction in the forward direction, decreasing the reactant concentration and increasing the product concentration.
The value of Kc increases correspondingly.

Question 21

effect of increasing temperature on the value of Kc for an endothermic reaction

Answer 21

Increasing the temperature drives the reaction in the forward direction, increasing the product concentration and decreasing the reactant concentration.
The value of Kc increases correspondingly

Question 22

effect of decreasing temperature on the value of Kc for an endothermic reaction

Answer 22

Driving the temperature drives the reaction in the reverse direction, decreasing the product concentration and increasing the reactant concentration
the value of Kc decreases correspondingly

Question 23

describe the effect of a change in concentration on the value of Kc

Answer 23

when concentrations are changed the equilibrium shifts position so as to return to the original Kc value.
the shift in position is directed by the value of Kc being restored.

Question 24

effect of pressure on Kc value

Answer 24

no change

Question 25

effect of catalyst on Kc value

Answer 25

no change

Question 26

if value of Kc<1,

Answer 26

equilibrium - almost entirely reactants
almost no reaction

Question 27

if value of Kc>1,

Answer 27

equilibrium - almost entirely products
proceeds virtually to completion

Question 28

if value of Kc=1

Answer 28

equilibrium - roughly in the centre
both reactants and products present

Question 29

Define the reaction quotient, Q

Answer 29

measures the relative amount of products and reactants present during a reaction at a particular point in time

Question 30

Q=

Answer 30

the equilibrium expression with non equilibrium concentrations:
aA + bB <-> cC + dD

Qc = [C]^c[D]^d/[A]^a[B]^b

Question 31

if Q>Kc

Answer 31

the concentration of products is greater than at equilibrium and the reverse reaction is favoured until equilibrium is reached

Question 32

if Q<Kc

Answer 32

the concentration of reactants is greater than at equilibrium and the forward reaction is favoured until equilibrium is reached

Question 33

if Q = Kc

Answer 33

the system is at equilibrium and the forward and reverse reactions occur at equal rates

Question 34

describe the relationship between Kc and Gibbs free energy change

Answer 34

at a given temperature, a negative ΔG value for a reaction indicates that the reaction is spontaneous and the equilibrium concentrations of the products are larger than the equilibrium concentrations of the reactants. Kc>1. The more negative the value of ΔG, the more the forward reaction is favoured and the larger the value of K.

Question 35

K = 1

Answer 35

at equilibrium, neither reactants nor products are favoured. ΔG = 0

Question 36

K>1

Answer 36

products favoured, ΔG <0 (negative value)

Question 37

K<1

Answer 37

reactants favoured, ΔG>0 (positive value)

Question 38

give two quantitative relationships between standard Gibbs free energy change, temperature, and the equilibrium constant

Answer 38

ΔG = -RTlnK

lnK = -ΔG/RT